I gave the under slung heel factor quite a bit of thought.
Long toes do not always cause under slung heels. I have seen horses with much too long feet/toes and perfectly upright heels.
What I also noticed is that under slung heels seem always to go hand in hand with either visible flaring of the toe or at least white line stretching in this area, often only indicated by a low toe angle.
So my conclusion is that the hoof being a capsule responds to applied forces as a whole, and when/if the laminar suspension is very strong and the wall material thick and uniform around the entire capsule not much is going to happen, but when the connection is weakened, the horn not strong all the way around or just plain weak and unhealthy then the toe lever starts to pull the whole hoof capsule forward as can be seen by the direction of the tubules. In the process contraction in the heels occurs as the whole foot loses its round shape, and the bars collapse and bend.
In order to fix the problem I feel that the only thing that needs to be done is to remove the toe lever and allow the horse to grow a new hoof capsule to ensure better laminar connection. The contraction will remedy itself, without any interference necessary from our part as the entire hoof capsule retracts to its proper place and will become round once more. Of course, the heels need to be kept low which goes without saying, but any forced opening and/or thinning are, in my opinion, more trouble than anything else.
I found that in order to remove the toe lever several possibilities are available, but I also found that, depending on the health of the laminar connection, some are more troublesome to the horse than others.
One way of doing it is to apply the true 4 point trim where the toe callous sometimes does get invaded. By this I mean that the break over is moved to the correct place disregarding the toe callous. This can work on strong walled feet and on soft ground but will otherwise make horses often sore.
Another way is to chop off the toe vertically into or even through the white line which some horses tolerate well and which makes for quick progress.
Yet another way is to rasp the toe wall thin, in a straight line from the coronary band, and let it meet the white line on the bottom, with the idea that new growth will be guided in a better direction.
What I have been doing, lately on my foundered horses is to just bevel the toe upward from the back of the white line as I found that all other ways seemed to make these horses invariably sore, and leave all flares, slippers etc. as they are. This is very much against what most any other expert would recommend for feet like this but it seems to work for my horses. Every time I do get tempted to remove at least some of the apparently excess wall I lose concavity and the horses get sore.